1. Field of the Invention
The present invention generally relates to isolation technology for semiconductor integrated circuits. In particular, the present invention relates to a method for fabricating trench-isolation structures. The trench-isolation structure in accordance with the present invention can minimize bird's beak and oxidation stress occurring in the conventional LOCOS method, and can further decrease plasma-caused damage to substrates near active regions.
2. Description of the Prior Art
Because remarkable advances were made in semiconductor integrated circuit fabrication technology, device dimensions have been decreased from sub-micron to half-micron. In the nearest future, the device dimensions of 0.35 .mu.m, 0.25 .mu.m, 0.18 .mu.m, or even 0.13 .mu.m will be successively matured to industrial applications. The smaller the device dimensions, the greater the number of devices can be accumulated on a single chip. However, as the device dimensions approach the deep sub-half-micron region, the demand on the isolation structures for reliably isolating neighboring devices increases.
As disclosed in N. Shimizu, et al., "Poly-Buffer LOCOS Process for 256 Mbits Cells," IEDM-92, pp.279-282, field oxide is thermally grown by means of wet oxidation at temperature of around 1000.cent.XC, and nevertheless typically made to seven to ten times thicker than the gate oxide in active regions. The field oxide grows where there is no masking nitride, but at the edges of the nitride, some oxidant also diffuses laterally. This causes the oxide to grow underneath and lift the nitride edges. Because the shape of the oxide at the nitride edges is that of a slowly tapering oxide wedge, it has been named a bird's beak. This bird's beak encroachment limits the scaling of channel widths. In addition, the step of forming thick field oxide creates another problem during the wet oxidation process--intensifying mechanical stress at the bird's beak.
A. Bryant, et al., "Characteristics of CMOS Device Isolation for the ULSI Age," IEDM-94, pp.671-674 discloses a trench isolation to solve those aforementioned problems, such as bird's beak encroachment and local mechanical stressing. However, the step of forming trenches during plasma etching processes is vulnerable to causing plasma damage to the substrate near active regions and therefore induces a great number of defects.